It is often necessary to maintain the position of a marine vessel. This can be done using the vessel's thrusters. Dynamic positioning systems are used to control the thrusters to apply thrust to the vessel. More particularly, the thrusters are controlled to apply thrust of the correct magnitude and in the correct direction in order to maintain the position and orientation of the vessel. In order to calculate the required thrust, it is necessary for dynamic positioning systems to measure or estimate environmental forces acting upon a vessel. In a conventional dynamic positioning system, estimates of environmental forces are made from the measurements made using position reference sensors, wind sensors, motion sensors, gyro compasses and other similar means.
Typically, a dynamic positioning system will control the position and orientation of a vessel with respect to surge, sway and yaw or heading. As will be readily understood, the environmental force acting on a vessel with respect to the surge and sway of the vessel will be linear forces whilst the environmental force acting on a vessel with respect to the heading or yaw of the vessel will be a turning moment. For the purposes of this specification it is to be understood that the environmental forces acting on a vessel in the following description include linear forces acting on a vessel with respect to the surge and sway of the vessel and a turning moment acting on the vessel with respect to the heading or yaw of the vessel. However, as will be readily understood by a person skilled in the art, the description is not limited to these degrees of freedom of a vessel and may be relevant to any of the six degrees of freedom of a vessel.
Jack-up vessels are vessels that have a plurality of long support legs that can be vertically lowered or raised. When the support legs are raised jack-up vessels can operate as marine vessels in a substantially normal manner. When the support legs are lowered jack-up vessels can be raised from the sea bed and supported out of the water on the support legs and thereby form secure platforms for operations. Jack-up vessels are of particular use for the installation and maintenance of offshore wind turbines and other similar offshore structures.
Before and after operations, when jack-up vessels are unsupported by the support legs, dynamic positioning systems are often used to move jack-up vessels into and out of position. However, during operations the main body of a jack-up vessel is raised out of the water and the vessel is supported on the sea-bed by the support legs. In this position the jack-up vessel will be substantially stationary. Being raised out of the water makes it impossible for a dynamic positioning system to estimate the environmental forces in a conventional manner. Thus when the operations are completed and the vessel is being lowered back into the water before the vessel transitions from being supported by the support legs to being floating in the water the dynamic positioning system will not be able to calculate the environmental forces acting on the vessel. Without knowing the environmental forces acting on the vessel the dynamic positioning system will be incapable of determining an appropriate thrust to apply to the vessel when the support legs are raised and no longer support the vessel. This can result in a vessel undergoing significant positional excursions due to the environmental forces acting on the vessel before it is possible for the dynamic positioning system to accurately estimate the environmental force and thereby control the thrusters to apply an appropriate thrust to the vessel. In this situation previous measurements of environmental forces are not suitable for use as it is almost inevitable that environmental conditions will not be completely unchanged from when the vessel was raised out of the water.
In light of the above, there is a need for a method of estimating the environmental forces acting on a supported jack-up vessel before the vessel transitions from being supported by the support legs to floating on the water in which it is situated.